1. Field of the Invention
The present invention relates generally to a rotor and more particularly to a rotor with reduced aerodynamically generated impulsive noise.
2. Description of the Prior Art
Sounds generated aerodynamically are sound fields which, instead of owing their existence to vibrations of solids, are rather by-products of airflows. One of the most serious problems in the development of a helicopter transportation system for civilian or military requirements is the noise generated by these vehicles due to aerodynamic mechanisms inherent in their design. A prominent and impulsive sound occurs when a helicopter operates in certain conditions. This impulsive sound is usually called blade slap or impulsive noise and it causes an observer in the acoustic far field to experience a high-level acoustic energy burst for each revolution of each helicopter blade. When impulsive noise occurs it is offensive and dominates all other helicopter noise sources. Helicopter designs, particularly in the past ten years, have increasingly called for high main and tail rotor tip speeds in order to minimize total vehicle weight and maximize payload. The combination of high hovering tip speeds and high forward velocities give rise to the impulsive noise problem. In the past, attempts have been made to reduce impulsive noise by altering the rotor tip shape or lowering the rotor tip speed. Although some impulsive noise reduction can be achieved by thinning the rotor tip, structural and aerodynamic constraints prevent the tips from being thinned enough to significantly reduce impulsive noise.
Previous rotor modifications aimed at alleviating blade slap have concentrated only on the very tip of the blade. We have found that at least the outer 30% of the rotor significantly contributes to rotor impulsive noise. For example, the rotor disclosed in U.S. Pat. No. 3,721,507 (Robert A. Monteleone) has a tip that is linearly and rearwardly swept in order to alleviate blade instability at high speed flight. Although Monteleone claims that noise reduction also results from the swept tip, our research indicates that the linearly swept tip actually generates two, rather than one, acoustic energy pulses per blade revolution.
Lowering tip speed has been the only truly effective method of lowering impulsive noise levels. This method has been used throughout the helicopter industry to attempt to meet the low noise levels required for military and commercial applications. The tradeoffs for rotor tip speed reduction are reduced rotor performance and decreased helicopter productivity--tradeoffs that helicopter users wish to avoid.